Finally i give up and ask Microsoft PSS for support, surprisingly the bug is quite simple: The boot image that VMM is using for offline conversion cannot fit into the 100MB default (classic install) system partiton [just a side note: the system partition is used for booting the system = boot sector, bootmgr, while the Boot partition contains Windows files]. In order to fix the issue:

Open an elevated command prompt and run : bcdboot c:windows /s c:

This will copy the required boot files to the C drive. Notice the hidden boot folder on the root of the C drive and also the bootmgr file.

Set the C drive as active : You can do this from disk management or using diskpart

Reboot the server

Open disk management and make sure the C drive is marked as SYSTEM.

Try the P2V process again and this time it should work fine

Obtained from this link http://www.rozmazat.cz/articles/2011/08/22/vmm-2008-r2-sp1-error-3140-when-trying-to-offilne-p2v-ws2008r2.html